Megachilidae bees are important pollinators in the Neotropical region, however information on the ecology and behavior of these species is still scarce. The objective of this study was to analyze the nesting biology of sympatric species in the União Biological Reserve, a remnant of Atlantic Forest in the southeastern Brazil. Our results indicated the occurrence of 17 species, representing significant richness compared to other areas in the Atlantic Forest. Five sympatric species built ¾ of all nests found and the architecture of their nests was studied including, for the first time, nests of a species of Megachile (Ptilosarus). The use of petals or leaf fragments in the construction of the nests was observed for species of Megachile (Chrysosarus), confirming previous data. The nesting activity period in the trap-nests occurred mainly in the rainy season, with different peaks among the species. The results indicate that the distinctive characteristics of the species, such as the type of material used in the nests, the dimensions of cavities, and the asynchronous nesting period, could be important for the niche differentiation of these sympatric species, allowing the maintenance and the survival of the most abundant Megachilidae populations in the area
